U.S. patent number 4,731,310 [Application Number 06/896,690] was granted by the patent office on 1988-03-15 for cathodic electrode.
This patent grant is currently assigned to W. R. Grace & Co.. Invention is credited to Menahem Anderman, Joseph T. Lundquist, Jr..
United States Patent |
4,731,310 |
Anderman , et al. |
March 15, 1988 |
Cathodic electrode
Abstract
A polymer bonded sheet product suitable for use as a cathodic
electrode in a non-aqueous battery system wherein the cathodic
electrode is a microporous sheet composed of from 6-10 weight
percent polyethylene having a molecular weight of 200,000 to
500,000, 90-94 weight percent of titanium disulfide particulate
material and from 0 to 2 weight percent of a plasticizer for the
polyethylene; the sheet having a void volume of from 15 to 25
percent.
Inventors: |
Anderman; Menahem (Boyds,
MD), Lundquist, Jr.; Joseph T. (Jessup, MD) |
Assignee: |
W. R. Grace & Co. (New
York, NY)
|
Family
ID: |
25406646 |
Appl.
No.: |
06/896,690 |
Filed: |
August 15, 1986 |
Current U.S.
Class: |
429/217;
429/212 |
Current CPC
Class: |
H01M
4/625 (20130101); H01M 4/621 (20130101); H01M
4/5815 (20130101); H01M 4/581 (20130101); H01M
4/136 (20130101); H01M 10/052 (20130101); H01M
4/62 (20130101); H01M 50/40 (20210101); Y02P
70/50 (20151101); H01M 10/0587 (20130101); Y02E
60/10 (20130101) |
Current International
Class: |
H01M
4/62 (20060101); H01M 10/36 (20060101); H01M
4/02 (20060101); H01M 4/58 (20060101); H01M
2/14 (20060101); H01M 10/40 (20060101); H01M
004/36 () |
Field of
Search: |
;429/217,212,194 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
NASA Technical Brief, vol. 9, No. 2, Item 103, (Summer, 1985).
.
Electrochemica Acta, vol. 29, No. 11, pp. 1589-1596, (1984). .
J. Electrochem Soc., pp. 656-660, (May, 1974). .
J. Electrochem Soc., pp. 1107-1109, (May, 1983)..
|
Primary Examiner: Niebling; John F.
Assistant Examiner: Marquis; Steven P.
Attorney, Agent or Firm: Troffkin; Howard J.
Claims
We claim:
1. A cathodic electrode suitable for use in a non-aqueous battery
system comprising at least one substantially homogeneous,
microporous sheet product having a void volume of from 15 to 25
volume percent and having a composition consisting essentially of
from about 90-94 weight percent of particulate material consisting
essentially of titanium disulfide having an average particle size
of less than about 20 microns, from about 6-10 weight percent of
high density polyethylene having a weight average molecular weight
of from about 200,000 to 500,000 and from 0 to about 2 weight
percent of an organic plasticizer for said polyethylene; and a
current collector composed of a conductive material, said collector
being in intimate contact with each of said at least one
microporous sheet product.
2. The electrode of claim 1 wherein the current collector is in the
form of a screen, grid, expanded metal, foil or woven or non-woven
fabric formed from carbon or a conductive metal.
3. The electrode of claim 1 wherein the polyethylene has a weight
average molecular weight of from about 200,000 to about 300,000,
the void volume is from 15 to 22 volume percent, the polymer is
present in 7 to 9 weight percent; the titanium disulfide is present
in from 91 to 93 weight percent; and the plasticizer is present in
0-1 weight percent.
4. The electrode of claim 2 wherein the polyethylene has a weight
average molecular weight of from about 200,000 to about
300,000.
5. The electrode of claim 1 composed of two sheet products and
having the current collector therebetween; said sheet products
forming a substantially unitary structure of total thickness of
less than about 50 mils.
6. The electrode of claim 2 composed of two sheet products and
having the current collector therebetween; said sheet products
forming a substantially unitary structure of total thickness of
less than about 50 mils.
7. The electrode of claim 3 composed of two sheet products and
having the current collector therebetween; said sheet products
forming a substantially unitary structure of total thickness of
less than about 50 mils.
8. The electrode of claim 4 composed of two sheet products and
having the current collector therebetween; said sheet products
forming a substantially unitary structure of total thickness of
less than about 50 mils.
9. In a secondary non-aqueous battery comprising at least one pair
of electrodes composed of anodic electrode and a cathodic electrode
and a non-aqueous electrolyte composition, said anodic electrode
formed from an alkali metal, wherein the improvement comprises
having the cathodic electrode consist essentially of the product of
claim 1.
10. In a secondary non-aqueous battery comprising at least one pair
of electrodes composed of anodic electrode and a cathodic electrode
and a non-aqueous electrolyte composition, said anodic electrode
formed from an alkali metal, wherein the improvement comprises
having the cathodic electrode consist essentially of the product of
claim 2.
11. In a secondary non-aqueous battery comprising at least one pair
of electrodes composed of anodic electrode and a cathodic electrode
and a non-aqueous electrolyte composition, said anodic electrode
formed from an alkali metal, wherein the improvement comprises
having the cathodic electrode consist essentially of the product of
claim 3.
12. In a secondary non-aqueous battery comprising at least one pair
of electrodes composed of anodic electrode and a cathodic electrode
and a non-aqueous electrolyte composition, said anodic electrode
formed from an alkali metal, wherein the improvement comprises
having the cathodic electrode consist essentially of the product of
claim 4.
13. In a secondary non-aqueous battery comprising at least one pair
of electrodes composed of anodic electrode and a cathodic electrode
ad a non-aqueous electrolyte composition, said anodic electrode
formed from an alkali metal, wherein the improvement comprises
having the cathodic electrode consist essentially of the product of
claim 5.
14. In a secondary non-aqueous battery comprising at least one pair
of electrodes composed of anodic electrode and a cathodic electrode
and a non-aqueous electrolyte composition, said anodic electrode
formed from an alkali metal, wherein the improvement comprises
having the cathodic electrode consist essentially of the product of
claim 6.
15. In a secondary non-aqueous battery comprising at least one pair
of electrodes composed of anodic electrode and a cathodic electrode
and a non-aqueous electrolyte composition, said anodic electrode
formed from an alkali metal, wherein the improvement comprises
having the cathodic electrode consist essentially of the product of
claim 7.
16. In a secondary non-aqueous battery comprising at least one pair
of electrodes composed of anodic electrode and a cathodic electrode
and a non-aqueous electrolyte composition, said anodic electrode
formed from an alkali metal, wherein the improvement comprises
having the cathodic electrode consist essentially of the product of
claim 8.
Description
BACKGROUND OF THE INVENTION
The present invention is directed to a highly filled TiS.sub.2 -
polymer bonded electrodes useful in a non-aqueous battery and to a
battery system containing said electrodes.
Storage batteries have a configuration composed of at least one
pair of electrodes of opposite polarity and, generally, a series of
adjacent electrodes of alternating polarity. The current flow
between electrodes is maintained by an electrolyte composition
capable of carrying ions across electrode pairs.
Non-aqueous batteries have certain distinct advantages over other
types of storage batteries. They use, as anodes, light weight
metals, such as lithium, lithium-aluminum alloys and the like which
are at the far end of the electromotive series. These batteries
have the potential for providing much higher gravimetric and
volumetric energy densities (capacity per unit weight and volume,
respectively) than other types of batteries, due to the low atomic
weight of the metal and high potential for forming a battery in
conjunction with suitable positive electrodes far removed from the
light weight metal (alkali metal) electrode (the description herein
will use batteries having lithium as the light weight metal anode
although other light weight metals can be used) in the
electromotive series. The battery can be formed in any conventional
physical design, such cylindrical, rectangular or disc-shaped
"button" cells, normally of a closed cell configuration.
The battery components of positive electrode, negative electrode
and separator can be in the form of distinct alternating plates in
a sandwich design or of a continuous spirally wound design as are
well known. The anodic electrodes can be formed, for example, from
lithium metal or its alloys on a support, such as a nickel coated
screen. The electrolyte can be formed of a non-aqueous solvent of
fused or solid electrolyte. Illustrative of known useful
non-aqueous solvents include acetonitrile, tetrahydrofuran and its
derivatives, propylene carbonate, various sulfones and mixtures of
these solvents containing a light metal salt such as lithium salts
as, for example, lithium perchlorate, iodide or hexafluroarsenate
and the like. An additional, normally passive component of the
battery is a separator membrane located between plates of opposite
polarity to prevent contact between such plates while permitting
electrolytic conduction. Separators are normally of the form of
sheets which possess very low electronic conductivity.
Significant developments have been made in the fabrication of
non-aqueous batteries. However, one of the major concerns is the
lack of development of a suitable cathdoe in which the
electrochemically cathodic material is present in the form of a
porous, flexible, sheet material. The cathodic active material must
be bonded into a unitary sheet by a material which is inert with
respect to the other components of the battery as well as being
inert and compatable to the active material. The bonding material
must be capable of readily forming a uniform sheet in which the
active material is uniformly distributed throughout the length and
breadth of the sheet as well as across its thickness to provide
maximum effectiveness. The bonding material must be kept to very
low amounts of the total sheet material or the cathodic active
material will be encompassed by the material and thereby
dramatically reduce the conductivity and activity of the resultant
cathodic sheet product. Even though present in only small amounts
the bonding polymer must be capable of maintaining the sheet
integrity and provide resistance to fractures, spalling and
disintegration attributable to the expansion and contraction forces
encountered in charge-discharge cycling when used in a secondary
battery system.
Polymer bonded electrodes presently known have a number of
deficiencies which has limited their utility and, thereby limited
the acceptance of an effective non-aqueous battery system. The
presently known polymer-bonded electrodes are not capable of being
mass produced by a reliable, cost-effective, non-aqueous process.
In addition, the majority of known polymer-bonded electrodes
exhibit flaking and disintegration when the formed sheet is further
processed such as when applied to a current collector and/or during
assembly into a battery.
A number of bonding polymers have been considered for and used in
the fabrication of cathodic polymer bonded electrodes. The most
widely used material at the present time is
poly(tetrafluoroethylene), commonly referred to as PTFE or by the
tradename Teflon. PTFE bonded electrodes have certain drawbacks
which limit their usefulness and ability to provide a highly
effective product. For example, the chemical inertness of this
polymer causes the fabrication of electrodes to be both difficult
and laborious. Generally, it requires initially mixing the active
material with an aqueous slurry of PTFE which is then doctored onto
a surface and heated to high temperatures (250.degree.-400.degree.
C.) to remove the water and cause bonding. The presence of water
and the processing at high temperatures limits the active materials
which can be used in forming the electrode product. For example,
titanium disulfide, a desirable active material, is known to be
unstable in the presence of water. PTFE bonded sheets tend to flake
and are not free standing unless large amounts of polymer are used.
The sheets are conventionally bonded to a current collector screen
by pressing them together at high temperatures. This process
normally produces a brittle product which tends to crack and chip.
Finally, a major defect of this known class of product is its
non-uniformity both in distribution of active material and of
porosity. This defect is inherently due to the processing
techniques required, especially the evaporation of solvent from the
materials causing non-uniformity across its thickness as well as
from point-to-point on the sheet product. Patents illustrating
formation of polymer bonded electrodes by this technology are U.S.
Pat. Nos. 3,457,113; 3,407,096; and 3,306,779.
Some work has been done to form a product from dry
tetrafluoroethylene suspensions to overcome the incompatibility
problems associated with water but such products require sintering
at very high temperatures (e.g. 400.degree. C.) which also limits
the types of active fillers which can be used. Patents illustrating
this known technology are U.S. Pat. Nos. 3,184,339 and
3,536,537.
More recently polymer bonded electrodes have been formed from
slurries of EPDM (ethylene-propylene-diene terpolymer) in an
organic medium, such as cyclohexane (see "Elastomic Binders for
Electrodes" by S.P.S. Yen et al., J. Electrochem. Soc., Vol. 130,
No. 5, Pg. 1107). Other elastomeric polymers, such as sulfonated
ionomers, butyl rubbers and the like have also been used in forming
electrodes by a slurry technique (See U.S. Pat. No. 4,322,317). The
resultant electrode products formed in this manner exhibit greater
elasticity and compatability with the other battery components.
However, the defects of non-uniformity of product, poor control of
porosity and pore size distribution remain a problem. In addition,
electrodes made by this method exhibit severe loss of activity
after being subjected to only a few charge-discharge cycles as
noted by the low figure of merit reported in U.S. Pat. No.
4,322,317.
It is highly desired to be able to provide a polymer bonded
electrode which exhibits high charge density; which is capable of
sustaining high discharge rates; and which is capable of exhibiting
very low capacity loss upon charge-discharge cycling. In addition,
the elctrode should be capable of being easily fabricated,
exhibiting a high degree of uniformity, being flexible material
which can be readily formed into desired configuration and
maintaining its integrity under the conditions encountered in a
battery (including expansion-contraction of cycling). Finally, it
is highly desired to provide a polymer-bonded electrode which is in
the form of a sheet of controlled microporosity capable of
permitting entry and mobility of electrolyte therein which can
thereby increase the electrode's activity.
Upon initial consideration, it might be assumed that many binding
materials could be used as alternatives to the small number of
materials presently used and obtain the desired results. However,
although there are a large number of polymers available as binders
in many applications including as electrode binders, a selection of
a specific binder is not obvious to the artisan when attempting to
provide a TiS.sub.2 filled cathodic electrode because of the many
factors which influence the results one obtains with any particular
binder. Among the major factors which influences the results
obtained are: (1) the solubility of the binder in the organic
electrolytes which are required in this application; (2) the
chemical stability of the polymer at the electrode potential
realizing that many cells are operated at different potentials; (3)
the stability of the electrochemically active and electrically
conductive materials used in combination with a particular binder
and under the conditions needed for fabrication; (4) the ability of
the polymer to bind the titanium disulfide and other particulate
material into a unitary structure at very low concentrations in
order to provide a cathodic electrode with good performance; (5)
the ability and ease of obtaining a uniform distribution of the
binder with the active material of the electrode; (6) the ability
of the polymer to maintain a stable cathodic electrode capable of
undergoing a multiplicity of charge-discharge cycling; (7) the
number and ease of the steps required to obtain the desired
cathodic electrode; and (8) the safety, availability of material
and cost. Thus, selection of a polymer for use in forming a high
performance electrode which contains titanium disulfide has been a
difficult task because of the above factors which impose severe
restrictions and limitations.
In a copending U.S. Application Ser. No. 843,347 filed Mar. 24,
1986, applicants disclosed a polymer bonded TiS.sub.2 electrode
which preferably contained ultrafine conductive carbon particulate
to enhance the electrical conductivity of the resultant sheet.
However, the use of such high surface area conductive carbon
reduces the charge density of the electrode and thus reduces the
capacity of the battery system. In addition, in some instances the
presence of carbon may cause electrolyte decomposition which
shortens the life of the battery. The presently described sheet
product provides a cathode having high electrical conductivity and
electrochemical activity without the difficulties associated with
the addition of conductive diluents such as carbon.
It has now been discovered that a cathodic polymer bonded electrode
suitable for use in non-aqueous batteries can be readily formed in
a manner which provides a superior electrode and overcomes the
processing problems associated with Teflon and other presently used
polymers as described above and can exhibit the highly desired
features of having high charge density, being capable of sustaining
high discharge rates and having very low capacity loss upon
charge-discharge cycling.
SUMMARY OF THE INVENTION
The present invention is directed to a TiS.sub.2 polymer bonded
electrode and to a non-aqueous battery system containing said
electrode product in which the electrode is a thin, microporous
sheet composed of from 6-10 weight percent polyethylene of a weight
average molecular weight of from 200,000 to 500,000, 90-94 weight
percent of titanium disulfide and from 0-2 weight percent of an
organic plasticizer for the polyethylene. The sheet is prepared by
initially forming a substantially uniform mixture of the components
with excess plasticizer, shaping the mixture into a sheet,
extracting a portion of the plasticizer, compressing the sheet and
then extracting the remainder of the plasticizer. The resultant
product is a flexible sheet material which possesses a high degree
of mechanical integrity, strength and uniformity, has a controlled
pore volume of from 15 to 25 percent with pore size of narrow
distribution and exhibits high conductivity of at least 0.15
reciprocal ohm-cm and preferably at least 0.3 reciprocal
ohm-cm.
The polymer bonded electrode product formed according to the
present invention is capable of exibiting very low loss of capacity
even after subjection to a large number of charge-discharge
cycles.
DETAILED DESCRIPTION OF THE INVENTION
The polymer bonded electrode product of the present invention is in
the form of a thin sheet which is required to be formed from a
homogeneous admixture of polyethylene, a plasticizer for the
polyethylene, and titanium disulfide. The electrode is not required
to contain additional electrically conductive materials (such as
carbon) to provide an electrode of high capacity.
The polymer electrode product of the instant invention is formed
through a series of precursor materials. Generally, a uniform
admixture is initially formed of polymer, plasticizer and titanium
disulfide. The admixture is capable of exhibiting sufficient flow
and rheological characteristics to permit the admixture to be
readily processed and shaped at relatively low temperatures (i.e.
25.degree. C.-170.degree. C). An initial sheet is formed from the
admixture. The plasticizer component is then partially removed from
the initial sheet. The initial sheet is then processed to provide
an intermediate sheet having a portion of the original plasticizer
content therein. The intermediate sheet is compressed prior to
final removal of the remaining plasticizer to provide a resultant
sheet having the desired void volume and TiS.sub.2 content. The
plasticizer removal normally occurs subsequent to the forming of a
laminate in which a metal screen (a current collector) is laminated
to a sheet or sandwiched between two sheets to provide an electrode
product. The final product, having had the plasticizer component
substantially removed, is composed of from 6 to 10 weight percent
of polyethylene and of from 90 to 94 weight percent TiS.sub.2. The
resultant product has a void volume of from 15-25 percent and is
useful as a polymer bonded electrode.
The present invention requires the utilization of polyethylene of
high density. The polyethylene should have a weight average
molecular weight of 200,000 to 500,000. Although homopolymers are
preferred the term "polyethylene", as used herein and in the
appended claims, shall mean polyethylene homopolymers and
copolymers in which copolymer is formed from olefinic monomers such
as ethylene, propylene, butene-1, acrylate and the like with the
major (preferably at least 80 percent) olefinic monomer being
ethylene.
The plasticizer of the instant composition must be present in the
initial formulating and processing to form an initial sheet
product, as more fully described below. The plasticizer provides
the means of fabricating the composition to a uniform consistency
and to aid in inducing and controlling the degree of porosity, the
pore size distribution and uniformity of porosity throughout the
resultant sheet product.
Plasticizers suitable for the instant invention are compounds which
are capable of plasticizing polyethylene, are substantially inert
with respect to titanium disulfide and are substantially soluble in
an organic solvent which is a non-solvent with respect to the
polymer component described above and the titanium disulfide.
Representatives of such plasticizers are organic esters, such as
sebacates, phthalates, stearates, adipates and citrates; epoxy
compounds such as epoxidized vegetable oil; phosphate esters such
as tricresyl phosphate; hydrocarbon materials such as petroleum oil
including lubricating oils and fuel oils, hydrocarbon resin and
asphalt and pure compounds such as eicosane; coumarone-indene
resins and terpene resins; tall oil and linseed oil. The preferred
plasticizers are hydrocarbon materials and most preferred
plasticizers are selected from petroleum oils. The plasticizer is
generally substantially free of water (anhydrous) and, therefore,
compatable with the subject battery system.
The organic plasticizer used herein aids in fabricating the sheet
product and in imparting microporosity to the resultant sheet. The
void volume of the resultant sheet will be directly dependent upon
the amount of plasticizer contained in the intermediate sheet prior
to subjecting the sheet to compression and to the amount of
plasticizer extracted therefrom to provide the final sheet product.
Void volumes of the final sheet product may range from 15 to 25
volume percent with from about 15 to 22 volume percent being
preferred. Electrodes with higher ranges of void volume have been
found to exhibit much higher capacity loss. The sheets void volume
is of a microporous character which generally have narrow pore size
distribution and are of low mean diameter (i.e. 0.01 to 0.5
microns) and can be determined by standard mercury intrusion
techniques.
The particulate material required in forming the present admixture
and the resultant sheet is composed of the cathodic
electrochemically active and electrically conductive titanium
disulfide. The term "electrochemically active" refers herein and in
the appended claims to the ability of the titanium disulfide to
enter and participate in a redox reaction with an alkali metal
during the operation and in the environment of an electrochemical
cell. The term "electrically conductive" refers herein and in the
appended claims to the ability of titanium disulfide to exhibit low
resistance to the passage of electrons. The particulate material
required in the present invention, titanium disulfide, when used in
the present sheet product configuration is capable of exhibiting
high electrochemical activity and electrical conductivity.
The titanium disulfide must be in particular form. The mean
particle size of the material should be 30 microns or less and
preferably 15 microns or less. Smaller particle size material is
preferred to enhance intimate contact between the particles of
electrochemically active material contained in the resultant
electrode. It has been unexpectedly found that by using titanium
disulfide and polyethylene in the amounts and of the type described
hereinabove in an electrode having the particular void volume of
from 15 to 25 volume percent one obtains an electrode which can
sustain its capacity over a large number of charge-discharge
cycling.
In addition to the above described components, the initially formed
admixture may further contain conventional stabilizers,
antioxidants, wetting agents, processing aids or mixtures thereof.
Representative of stabilizers are 4,4-thiobis(6-tertbutyl-m-cresol)
sold commercially under the tradename "Santonok" and
2,6-ditert-butyl-4-methylphenol sold commercially under the
trademane "Ionol". Examples of known commercially available wetting
agents include sodium alkyl benzene sulfonate, sodium lauryl
sulfate, dioctyl sodium sulfosuccinate, and isooctyl phenyl
polyethoxy ethanal. Processing aids include stearates, graphite and
the like.
The above-described components can be readily formed into a
substantially homogeneous admixture. The initial admixture should
be formed by blending from about 3 to 30 (preferably 12 to 20))
volume percent polymer, from about 27 to 76 (preferably 40 to 55)
volume percent of TiS.sub.2 and from about 20 to 70 volume percent
of polymeric plasticizer.
The blending of the components can be readily accomplished by
conventional means such as by initially mixing at room temperature
in a blender and then in a Banbury, Brabender or sigma blade mixer
or the like at moderate (about 25.degree. to about 170.degree. C.,
preferably from about 120.degree. to about 160.degree. C.)
temperatures. The blending and processing can be done under dry
conditions to avoid water pick-up by the materials.
It has been found that extremely high particulate content
admixtures required by the present invention exhibit rheological
properties which permit them to be readily shaped and formed into
thin sheet products of less than about 50 mils and preferably less
than about 20 mils. It must be understood that the particular
thickness can be customized by the artisan based on the battery
design and its acceptable drain rate. Sheet products and electrodes
therefrom can be made of less than 5 mils thickness. Sheet products
made by presently known conventional techniques can not be formed
of such thin dimensions and maintain good mechanical properties as
is attainable by the sheet products of the present invention. The
term "sheet" as used herein and in the appended claims refers to a
shaped product having extensive length and breath dimensions and of
thin cross-section and which may have major surfaces which are
substantially flat or of a predetermined design. The initial sheet
product can be readily formed from the admixture by subjecting the
admixture to extrusion, calendering, injection molding or
compression molding processing. All of these processing means are
capable of producing the initial sheet in large volume using low
labor involvement. The most preferred method is extrusion of the
admixture using a conventional extrusion apparatus to continuously
provide initial sheet product.
The forming of the initial sheet (a sheet having high levels of
plasticizer therein) can be readily accomplished at moderate
operating conditions, including low temperatures of from about
25.degree. to 175.degree. C. and preferably from about 120.degree.
to 160.degree. C. Such temperatures allow formation of sheet
product using components normally deemed unsuitable under known
slurry processes. Further the present process provides a sheet
which is a free-standing and has substantial uniform distribution
of particulate material throughout its length and breadth
dimensions as well as across its cross-sectional dimension.
The initially formed sheet can be formed into a final sheet product
suitable for use as a cathode through the formation and processing
of an intermediate sheet product. The formed initial sheet, as
described hereinabove contains a very high percentage of
plasticizer. Removal of substantially all of the plasticizer would
provide a sheet product highly loaded with TiS.sub.2 and having a
large void volume. It has been found that such sheet products do
not exhibit the ability to sustain its capacity over a large number
of charge-discharge cycling. However, when the final sheet is
formed as described herein, one achieves the desired product. Such
a final sheet can be achieved by processing an initially formed
sheet into an intermediate sheet having from about 10 to 22 volume
percent plasticizer therein. Such an intermediate sheet can be
formed by a variety of manners such as by (a) removing a portion of
the plasticizer contained in the initial sheet to reduce the
plasticizer content to between 10 and 22 volume percent; (b)
removing substantially all of the plasticizer contained in the
initial sheet and then causing a fixed amount of plasticizer to be
absorbed into the sheet to provide an intermediate sheet having
plasticizer content of from 10 to 22 volume percent; or (c) forming
the initial sheet from mixtures of a first and a second plasticizer
which have mutually exclusive solubility in two solvents useful for
extraction, removing the first plasticizer such as by extraction or
the like with a solvent which is a substantial non-solvent for the
second plasticizer therein to thus provide an intermediate sheet
having from 10 to 22 volume percent of the second plasticizer. The
intermediate sheet is then compressed such as by passing the sheet
through nip rollers or the like to cause the sheet to be
substantially nonporous. The compressed intermediate sheet is then
subjected to extraction or the like to remove substantially all of
the plasticizer contained in the intermediate sheet to provide a
resultant sheet product having 6 to 10 weight percent polyethylene,
90 to 94 weight percent TiS.sub.2 and from 0 to 2 weight percent
plasticizer as described hereinabove.
The formed sheet, either as the initial, intermediate or final
sheet, can be readily made into a suitable cathodic electrode by
laminating a conventional current collector with at least one sheet
of the present invention. The plasticizer component can be
extracted, as described below, prior or subsequent to lamination
with the current collector. It is preferred to form the laminate
structure of at least one sheet with a suitable current collector
prior to extraction of all of the plasticizer material. One
preferred mode is to form the laminate structure during the
compression of the intermediate sheet, as discussed above.
The current collector is normally a screen, grid expanded metal,
woven or non-woven fabric or the like formed from efficient
electron conductive materials such as carbon, or metals such as
copper, aluminum, nickel, steel, lead, iron or the like. The
current collector, when laminated to the final sheet product (a
sheet substantially comprising particulate material bonded by very
low amounts of polyethlyene) of the present invention, provides a
low electronic resistance path between the active material and the
battery terminal.
The sheet product, with or without the presence of plasticizer, is
a pliable and moldable material which can be readily laminated to
the collector screen by concurrently passing a screen and at least
one sheet through a set of nip rollers or the like to press (under
low pressure and preferably at moderate temperatures of about
25.degree. to 170.degree. C.) to produce a laminate product. It is
preferred that the laminate be of a configuration of a screen
sandwiched between (and thereby embedded in) two sheets although a
laminate of a single sheet and single screen may be desired in
certain applications. The laminate can be most readily formed by
utilizing an initial sheet product immediately after its production
to utilize the sheet in its elevated temperature state.
The plasticizer contained in the initial formed sheet should be
substantially completely removed by means of extraction using
suitable solvent. The composition of the resultant electrode will
depend upon the degree of extraction of the plasticizer. The
plasticizer can be substantially completely removed, leaving a
microporous polymeric sheet product which is highly filled with
titanium disulfide. The resultant sheet product exhibits good
physical properties and a high degree of microporosity. The
microporosity character of the resultant polymer bonded electrode
provides a means to permit the electrolyte to be in intimate
contact with a very high percentage of the titanium disulfide
particulate material. It is believed, although not meant to be a
limitation on the present invention, that the microporous structure
of the sheet permits the particles residing in the interior of the
sheet to be more active.
The procedure for extraction of the plasticizer from a sheet
product is well known and is not meant to form a part of the
present invention, per se. The solvent or extraction conditions
should be chosen so that the polyolefin and particulate material
are essentially insoluble. For example, when petroleum oil is to be
extracted from the formed sheet, the following solvents are
suitable; chlorinated hydrocarbons, such as trichloroethylene,
tetrachloroethylene, carbon tetrachloride, methylene chloride,
tetrachloroethane, etc., as well as hydrocarbon solvents such as
hexane, benzene, petroleum ether, toluene, cyclohexane, gasoline,
etc. Aqueous solutions should not be used as these would react and
decompose the particulate material used in the instant electrode
product.
The extraction temperature can range anywhere from room temperature
up to the melting point of the polyolefin as long as the polyolefin
does not dissolve. The temperature can be maintained such that all
components remain stable and are not adversely effected.
The time of the extraction will vary depending upon the temperature
used and the nature of the plasticizer being extracted. For
example, when a higher temperature is used, the extraction time for
an oil of low viscosity can be a very short time of up to only a
few minutes, whereas if the extraction is performed at room
temperature, the time requirement will be greater.
The final composition of the polymer-bonded electrode sheet product
will depend upon the original composition and the degree of
extraction of the plasticizer from the sheet product. The final
extracted sheet must have a composition comprising from 6 to 10
weight percent polyethylene, about 90 to 94 weight percent titanium
disulfide, and from about 0 to 2 weight percent plasticizer. The
more preferred electrode comprise a mixture of from 7 to 9 weight
percent polyolefin, 91 to 93 weight percent titanium disulfide, and
from 0 to 1 weight percent plasticizer.
The electrical conductivity of the resultant sheet products were
measured with a Yellow Spring Instrument Conductivity Bridge at 1
KHz placing a nickel metal clamp on each of the two opposite ends
of the specimen to be tested in such a manner as to have a free
sample spacing of 1 cm by 1 cm not covered by the clamps. The
thickness of the samples were measured. The clamps were connected
to the conductivity bridge and the resistance of the samples were
measured. To check the accuracy of the measurements, the clamps
were adjusted to a spacing of 2 cm by 1 cm and the resistance
remeasured.
The porosity volume percents or void volume percent were calculated
for the resultant sheet product by calculating the wet weight minus
dry weight divided by the sheet product's geometric wet volume.
Charge-Discharge cycling was performed on cells having the subject
sheet using a Princeton Applied Research Model 363 galvanostat. The
galvanostat was powered and monitored with an Analog Devices
.mu.MAC 5000 microcomputer which controlled the current passing
through the cell and measured the current passing through the cell
and measured the current voltage and charge throughout the
cycle.
The following examples are given for illustrative purposes only and
are not meant to be a limitation on the subject invention, except
as made in the claims appended hereto. All parts and percentages
are by weight unless otherwise indicated.
EXAMPLE I
(a) 8 parts of a high density polyethylene having a weight average
molecular weight of 250,000 were mixed with 21 parts of hydrocarbon
oil (Sunthene 255: density of 0.89 g/ml, 54 ssu at 210.degree. F.,
flash point of 390.degree. F.) and 84 parts of a commerically
available battery grade titanium disulfide having an average
particle size of 10 microns. The mixture was compounded in a
Brabender maintained at 150.degree. C. for two 10 minute periods.
The resultant homogeneous mixture was pressed into flat sheets
using a flat plate press (Wabash) maintained at 150.degree. C. at a
pressure of 400 psi to obtain sheets of 14.5 mils. thickness.
(b) An expanded Ni screen (5 mils. thick) having a nickel tab
attached to one end was placed adjacent to a sheet formed in the
manner described in paragraph (a) above. The composite was pressed
using a flat plate press (Wabash) maintained at 150.degree. C. and
500 psi pressure. The pressed product was observed to be a unitary
structure having the screen embedded within. The pressed sheet was
then immersed in cyclohexane bath for 15 minutes and then vacuum
dried. The porosity of the sheet was about 40 percent. This sheet
was then immersed in a 38 vol. percent solution of hydrocarbon oil
(Sunthene 255) in cyclohexane to have a fixed amount of oil
absorbed by the sheet. The sheet was removed from the solution and
dried to permit the cyclohexane to evaporate. The electrode was
pressed once more to remove the voids, then the oil was removed by
extraction as described above and finally dried.
The resultant electrode was composed of 8.7 weight percent
polyethylene and 91.3 weight percent titanium disulfide having a
pore volume of 20 vol. percent.
EXAMPLE II
The electrode of Example I above was placed in an Argon atmosphere
glove box and used to fabricate a spirally-wound Li-TiS.sub.2
battery cell. The cell was formed from lithium foil, commercial
microporous polypropylene separator, the electrode of Example I and
5 mil of electrolyte solution composed of 1.5 M LiAsF.sub.6 in
2-methyl tetrahydrofuran.
The solid components of the cell will fit into a standard AA size
cell with the electrolyte being in excess. The cell was sealed in
glass tubing, evacuated and then filled with the electrolyte
solution through a Ni tube which was subsequently sealed. The cell
contained 1.14 Ah of TiS.sub.2 and 1.80 Ah of Li.
The cell was discharged to 1.6 volts and charged to 2.6 volts. The
cell delivered 0.96 Ah (84% of theoretical) (1 mA/cm.sup.2) at 190
mA in cycle No. 7 and 0.68 Ah (63% of theoretical) at the same rate
at cycle No. 71.
EXAMPLE III
Sheets were formed in the same manner as described in Example I
above except that 8 parts of polyethylene were mixed with 19 parts
of hydrocarbon oil and 89 parts of TiS.sub.2. Two sheets were
positioned on each side of a nickel expanded screen and pressed
into a unitary sheet and then subjected to two fresh baths of
cyclohexane to extract substantially all of the oil. The electrode
sheet was then immersed in a 38 vol. percent solution of
hydrocarbon oil (Sunthene 255) in cyclohexane to allow sufficient
oil pick-up to form the desired product void volume. The sheet was
dried and pressed to remove excess voids as described in Example I
above and then the oil contained in the sheet is removed by
extraction with final drying.
A small rectangular glass sealed cell was built using the above
electrode with lithium foil and an electrolyte solution composed of
1.2 M LiAsF.sub.6 in 2-methyl tetrahydrofuran. The cell was
discharged to 1.6 V and charged to 2.6 V. The utilization of the
TiS.sub.2 electrode (% of theoretical) was 90 percent for cycle No.
3 (1 mA/cm.sup.2); 81 percent for cycle No. 28 (1.5 mA/cm.sup.2)
and 75% for cycle No. 65 (1.5 mA/cm.sup.2). The charge density of
this electrode was 0.9 mAh/cm.sup.2 -mil. The fade rate was very
low.
EXAMPLE IV
The following Examples are made for comparative purposes:
1. A sheet product was formed in the same manner as described in
Example III above except that only the initial extraction step was
performed. The resultant electrode sheet was composed of 8.3 weight
percent polyethylene and 91.7 weight percent TiS.sub.2 and had a
void volume of 36 percent.
The electrode sheet was used as the cathode component of a
rectangular cell fabricated in the same manner as described in
Example III. The charge density of the TiS.sub.2 electrode was
about 0.7 mAh/cm.sup.2 -mil. The cell was discharged (at 1
mA/cm.sup.2) to 1.6 volts and charged to 2.6 volts. The cell
delivered 92 percent of capacity at cycle No. 2 but only 60 percent
of capacity at cycle No. 45. The observed large cell capacity fade
was due to deterioration of the TiS.sub.2 cathode as the lithium
and electrolyte were present in excess.
2. 4 parts of polyethylene having a weight average molecular weight
of 5.times.10.sup.6 were mixed with 20 parts of hydrocarbon oil
(Sunthene 255) and 105 parts of commercially available battery
grade TiS.sub.2 having an average particle size of 10 microns. The
mixture was processed into an electrode in the same manner as
described in Example I above. The resultant electrode was used to
form a Li-TiS.sub.2 rectangular cell as described in Example III
above.
The cell was discharged at 1 mA/cm.sup.2 to 1.6 volts and charged
at 0.35 mA/cm.sup.2 to 2.6 volts. The TiS.sub.2 capacity
utilization was 95% for cycle No. 2; 71% at cycle No. 10; and 60%
at cycle No. 18. The large capacity fade was due to TiS.sub.2
electrode deterioration as lithium and electrolyte were in
excess.
* * * * *